* The estate in Ascot has been declared safe but death is being treated as ’unexplained’

The mystery surrounding the death of Russian oligarch Boris Berezovsky deepened last night after radiation experts spent hours combing the Berkshire mansion where his body was found.

A two-mile safety cordon was set up amid fears the exiled tycoon had been murdered with radioactive poisoning.

The alert had been raised when a paramedic’s radiation alarm was triggered as he left the £20million property after failing to revive Mr Berezovsky, whose body was discovered by a bodyguard on the bathroom floor.

The estate, in Ascot, was finally declared safe after an exhaustive search by experts in chemical, biological and nuclear emergencies.

Last night police said the death of the 67-year-old tycoon was ‘unexplained’, with the cause still unknown. A senior detective said he was keeping an ‘open mind’.

Mr Berezovsky was found dead on Saturday afternoon after the bodyguard had become concerned and smashed open the bathroom door.

Some friends said they believed Mr Berezovsky may have been the victim of a professional hit for speaking out against Russian president Vladimir Putin’s regime.

Others thought he had killed himself after becoming depressed over the loss of his wealth and status.

he controversial tycoon, who was once worth £3billion, was to be a key witness at the inquest of murdered spy Alexander Litvinenko, who was assassinated with radioactive poisoning in London six years ago.

Historian Yuri Felshtinsky, who has known Mr Berezovsky since 1998, said: ‘We do not have facts yet but we must bear in mind that there have been several questionable deaths of Russian emigres in the UK.

’It is more plausible to me that [Berezovsky] was killed as an act of revenge for speaking out against the Kremlin or perhaps as a warning to others not to cross them.

‘It is possible to kill someone and disguise it as natural causes. The FSB [the Russian security service] have structures in place to kill people. For a long time the British authorities believed Litvinenko was dying of some unknown disease. He did not have a heart condition, appeared to be in good health.’

Mr Berezovsky was said to have been ‘destroyed’ after losing a £3billion legal action against his former business partner, Chelsea Football Club owner Roman Abramovich.

He sued his one-time protege last year alleging that he had been intimidated into selling him shares in Russian oil conglomerate Sibneft. The legal battle is thought to have been the most expensive of all time, with Mr Berezovsky left with costs as high as £100million.

He had faced a new legal battle brought by his former girlfriend Elena Gorbunova, who had applied to the High Court to freeze his reported £200million assets.

Last night former Tory spin doctor Lord Bell, who was a friend, said Mr Berezovsky had lost all of his money and had even sold the house in Ascot recently to pay off his debts. He was understood to have sold properties on the French Riviera and an Andy Warhol print, Red Lenin, for £133,875.

‘I’m very sad,’ Lord Bell said. ‘I’ve lost a very great friend. He was depressed about his financial situation, his legal situation and his private life. And he was very badly battered by the judge’s comments in the case against Abramovich.’

But Lord Bell added: ‘I don’t happen to think that he was the kind of person who would commit suicide.’

Police and forensic teams descended today on the home of Russian oligarch Boris Berezovsky who was found dead in his multi-million pounds mansion, Titness Park in Ascot. Chemical, biological, radiological and nuclear trained officers searched his multi-million pound home. The exiled 67-year-old tycoon nicknamed the ’godfather of the Kremlin’, is reported to have been found by his bodyguard at his mansion.

However, in an informal interview with a Russian journalist on the night before he died, Mr Berezovsky reportedly said: ‘My life no longer makes sense. I have no desire to take part in politics. I don’t know what I should do. I am 67 years old and I don’t know what I should do from now on.’

The journalist, Ilya Zhegulev, claims the tycoon was desperate to return to Russia and hated being in exile.

The Kremlin claimed yesterday that Mr Berezovsky had written to Mr Putin several months ago to ask forgiveness and seek a route back to Russia. Lord Bell said such a suggestion was ‘nonsense’ and the oligarch remained as much as ever a target of the Russian state.

Mr Berezovsky, a former mathematician who had made his fortune in oil, aviation and television in the immediate aftermath of the fall of communism, fled Russia in 2000 and was put on the country’s official wanted list in 2001 on charges of fraud and money laundering.

The UK granted him political asylum in 2003 and he used London as a base from which to launch critical attacks on Mr Putin and to call for his overthrow.

He had survived several assassination attempts in Russia, including a bomb that decapitated his chauffeur.

Yesterday Detective Chief Inspector Brown, of Thames Valley Police, said: ‘We are at the early stages of the investigation and we are retaining an open mind as we progress.

‘The investigation team are building a picture of the last days of Mr Berezovsky’s life, speaking to close friends and family to gain a better understanding of his state of mind.

‘We do not have any evidence at this stage to suggest third party involvement.’